Henry Gantt, a major figure in the “scientific management” movement of the early twentieth century, developed the Gantt chart around 1917. A Gantt chart displays project activities as bars measured against a horizontal time scale. It is the most popular way of exhibiting sets of related activities in the form of schedules.
1. The Chart
Figure 5-19 shows a Gantt chart of the sample project in Table 5-4. The expected times are used in this illustration. Clearly, Gantt charts are easy to draw. Because task names are usually descriptive, each task shows its name, WBS number, or ID number in order to identify predecessors. Any activity that has no predecessors starts at the beginning of Day 1 and extends to its duration (as in task a). An activity with predecessors begins when its latest predecessor has been completed (as in task f or h).
Problems in understanding the chart can arise, however, when several tasks begin at the same time and have the same duration. If one such task is on the critical path and the others are not, it may be difficult to find the critical path on a Gantt chart. For instance, had c and d both been the same duration, it would not have been possible to tell which was predecessor to f and which to g, just by looking at the chart. This is only a problem when the Gantt chart is prepared manually. Most software, MSP included, will use arrows, bolded bar outlines, colored boxes, or some other visible means of marking the critical path on a Gantt chart as in Figure 5-20.
Even with software aid, the technical dependencies are harder to see on a Gantt chart than on a PERT/CPM network. On the network, technical dependencies are the focus of the model. As can be seen in Figure 5-20, information can easily be added to the chart to show such things as ES, EF, LS, LF, and slack.
Software such as MSP makes it easy to use a Gantt chart or network to view critical tasks and paths of a project. One can even experiment with adjustments to the project— play “what if” with the project schedule, immediately observing results of the experiments on the screen. At times the PM may question an estimate of task duration, or of
the a, m, and b time estimates, submitted by a member of the project team. It is simple to enter alternate time estimates and instantly see the impact on project duration. Of course, this can also easily be done using MSP’s PERT network and using simulation.
A great deal of information can be added to Gantt charts without making them difficult to read. A construction firm of our acquaintance added the following symbol to activities that were slowed or stopped because of stormy weather . They used other symbols to indicate late deliveries from vendors, the failure of local government to issue building permits promptly, and other reasons why tasks might be delayed. Milestone symbols—diamonds, 0, in MSP—are added to the charts, with different shading or color to differentiate between “scheduled” and “completed” milestones. MSP is limited only by the PM’s imagination in what can be shown on a network, Gantt chart, or in a project plan.
The major advantage of the Gantt chart is that it is easy to read. Such charts commonly decorate the walls of the project office (or “war room”). They can be updated easily. This is both the strength and the weakness of the Gantt chart. Anyone interested in the project can read a Gantt chart with little or no training—and with little or no technical knowledge of the project. This is the chart’s strength. Its weakness is that to interpret beyond a simplistic level what appears on the chart or to alter the project’s course may require an intimate knowledge of the project’s technology—not necessarily visible on the chart, but available on the network or the project plan. Not uncommonly, the Gantt chart is deceptive in its apparent simplicity.
We should add that one must be cautious about publicly displaying Gantt charts that include activity slack, or LSs and LFs. Some members of the project team may be tempted to procrastinate and tackle the work based on the LS or LF. If done, this makes a critical path out of a noncritical path and becomes an immediate source of headaches for the PM who, among other things, loses the ability to reschedule the resources used by tasks that once had slack. Senior managers have even been known to view activity slack as an invitation to shorten an entire project’s due date. We recommend caution and careful education of the boss.
At base, the Gantt chart is an excellent device to aid in monitoring a project and/or in communicating information on its current state to others. Gantt charts, however, are not adequate replacements for networks. They are complementary scheduling and control devices.
The Gantt chart is a useful complement to a project network. It is easily constructed and read. It can contain a considerable amount of information and is an excellent communication device about the state of a project. Its major weakness is that it does not easily expose the project’s technology, that is, the technical relationships between a project’s many activities. Even with predecessors marked on a Gantt chart, it is difficult to see the project technology and, thus, to use the Gantt chart alone to manage a complex project. PERT/CPM networks are often used as complements to Gantt charts.
Source: Meredith Jack R., Mantel Jr. Samuel J., Shafer Scott M., Sutton Margaret M. (2017), Project Management in Practice, John Wiley & Sons, Inc. 3th Edition.
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